Field of the Invention
This invention relates to an implantable drug delivery apparatus that can be actuated and refilled without removing a housing structure from a patient. The implantable drug delivery apparatus of this invention has relatively few moving mechanical components and thus is relatively inexpensive, reliable and safe.
Description of Prior Art
Existing implantable drug pumps operate with a single bellows and a relatively complex valve arrangement. Use of a single valve in such conventional implantable drug pumps is relatively unsafe because if the valve sticks in an open position, the patient receives an overdose of drug. Conventional implantable drug pump systems thus require multiple valves, motorized peristaltic mechanisms, roller mechanisms and/or other relatively complex mechanical mechanisms to assure positive closure of the discharge catheter after delivery of an intended dose of drug.
U.S. Pat. No. 5,382,236 discloses an implantable infusion pump with a flexible bellows medication container which is mounted within a sealed enclosure. A propellant gas acts upon the bellows medication container to force drug through a capillary tube.
Many conventional implantable drug pumps employ a bellows element mounted within a housing and a two-phase fluid or propellant occupies a void between the bellows and a housing. For example, U.S. Pat. No. 5,049,141 teaches an apparatus that uses a two-phase fluid to actuate a titanium diaphragm. U.S. Pat. No. 5,207,666 teaches a fluid metering device for an implantable drug delivery system which uses a bellows-type reservoir to deliver a drug to the metering device.
Two-phase or liquid/vapor fluids are used in conventional implantable infusion pumps for the purpose of maintaining a constant pressure on the drug so that the drug flows through a capillary at a relatively constant flowrate.